1. Field of the Invention
The present invention generally relates to the field of electro-acoustic transducer circuits. More specifically, the present invention relates to tuning circuitry for capacitive electrostatic microfabricated electro-acoustic transducers.
2. Description of the Related Art
An electro-acoustic transducer is an electronic device used to emit and receive sound waves. These transducers are used in medical imaging, non-destructive evaluation and other applications. Ultrasonic transducers are electro-acoustic transducers that operate at higher frequencies, typically at frequencies exceeding 20 kHz.
The most commonly used type of ultrasonic transducer is the piezoelectric transducer (PZT) made of ceramic materials. In recent years, a revolutionary, new technology has been developed with the potential of displacing conventional piezoelectric ceramic-based ultrasound transducers used for medical ultrasound imaging. These new transducers are made of fine microfabricated membranes suspended above Silicon-based substrates. These transducers operate in an electrostatic mode and electrically approximate a parallel-plate capacitor with finely spaced plates. These microfabricated transducers have considerable potential since the microfabrication process gives rise to low cost, highly complex structures—such as finely pitched 2D arrays of elements. Furthermore, since the microfabricated transducers are based on Silicon, it is envisioned that suitable driver and receiver circuitry may be integrated onto the same Silicon substrate or onto one immediately adjacent to the transducer substrate. Thus, the microfabrication technology may enable 2D arrays and real-time 3D imaging, which until now has been hampered by the cost and complexity of the cumbersome, time consuming, low-yield manufacturing processes required for the ceramic-based arrays. The microfabrication technology may also enable new intravascular applications such as placing transducer arrays on the tips of catheters or on other temporary, or semi-permanent, minimally invasive monitoring instrumentation used inside the body to monitor physiological functions (e.g., blood flow, blood pressure, etc.).
One drawback of the electrostatic microfabricated transducer arrays is that they substantially behave with the electrical characteristics of a capacitor. The capacitance of the microfabricated transducer introduces a negative reactance component to the overall transducer impedance, which makes the transducer inefficient. What is needed is a way to tune out the negative reactance of the microfabricated transducer. Further still, the practical constraints of making connections to the microfabricated transducer can introduce sources of parasitic capacitance that, in addition to the transducers intrinsic capacitance, add to the negative reactance of the impedance. Thus, it is also desirable to tune out parasitic components of the transducer assembly's capacitance.
A typical imaging probe used, for example, in harmonic imaging has between about 128 and 192 channels. Each transducer associated with this typical imaging probe operates in two directions (e.g., transmit and receive), or as is know in the art, in a pulse-echo fashion. Each transmit and receive pair operates using the same path signal path, typically a coax path (e.g., 192 coax paths for the 192 channel probe). The transmit voltages of such systems can be large, on the order of +/−100V, and are very short in duration, for example, 1 cycle of a 5-10 MHz sine or square wave. The receive voltages of such systems can be quite small, on the order of 100 mV down to less than 1 mV. To ensure the desired probe quality, receive sensitivity is an important design constraint, so low noise components are better. Any active circuitry attached to the transducer should be protected from large transmit voltages, while at the same time should be able to switch quickly into a highly sensitive receive mode (e.g., within about ½ μsec for the typical system).
The capacitive electrostatic transducer of the typical probe is a capacitive sensor that, when biased with a DC potential, acts as a transformer between the electrical and acoustic domains. The electrical impedance of this transducer can be dominated by a capacitive component on the order of 20-200 pF, which also contains a resistive part that varies with frequency and has real to imaginary ratios on the order of about 1:4 to 1:15 or more. FIG. 1 illustrates the complex series impedance vs. frequency plots for such a transducer.
The effect of a relatively large imaginary part compared to the real part of the transducer is that it can make transduction inefficient and it can cause reflections in the electrical and the acoustic domains. For example, it is typical for a coaxial cable to connect each active transduction element in the probe to the ultrasound system. Such cables have typical impedance ranges of 50 to 75 Ohms. A transmit signal traveling from the system to the probe tip on such a cable encounters the complex series impedance of FIG. 1. If the absolute magnitude of the complex impedance does not match the cable impedance, a reflection of the signal will bounce back toward the system. Furthermore, the voltage established across the transducer's impedance will be apportioned between the real and the imaginary part of the impedance. Only the real part of the impedance corresponds to the actual output of ultrasound in the medium of interest, so if the imaginary part is relatively large, only a small fraction of the transmit voltage is available for sound transmission.
From the acoustic domain's perspective, an ultrasound wave traveling in the medium (for example, the human body) toward the transducer will be partially reflected unless the acoustic impedance presented by the transducer is identical to that of the medium. Acoustic reflections are undesirable for two reasons. First, a reflection implies that not all of the power in the acoustic wave is being converted into an electrical signal, so transduction is not efficient. Second, and of particular importance in imaging applications, reflections from the transducer surface can manifest themselves as reverberation, or ghost images. A reflection from the transducer surface is in effect a false transmit event.
From systems theory, it can be demonstrated that a transducer will present a impedance matched to the medium if it is electrically connected to a receive circuit with an input impedance equal to the complex conjugate of the transducer's electrical impedance when loaded by the medium of interest. Thus, what is needed is electrical circuitry for presenting the desired impedance to the probe transducers.
It is possible, as shown in the embodiment of FIG. 5, to generate an impedance that looks like a negative reactance using passive components. However, passive designs are practical over a relatively narrow range of frequencies. Thus, what is needed is active circuitry for presenting a negative capacitance load to the probe transducers. As may be evident to those skilled in the art, negative capacitance circuits can lead to unstable conditions; it is desirous to achieve a negative capacitance that follows the transducer capacitance as closely as possible without inducing instability in the probe.
Furthermore, the active receive circuitry needs to be protected from the transmit signal. One approach is to switch the negative capacitor out of the circuit on transmit, and then switch it back immediately after transmit in preparation for receive. However, typical approaches for such protection circuitry require high voltage components given the +/−100 V transmit pulse.
In order to enable small probe packages and to limit the parasitic capacitance that loads the transducer on receive, it is desirable to provide active circuitry that in the form of an integrated circuit. However, low noise integrated circuit processes that are viable candidates for negative capacitor and pre-amplification circuits are not usable for high voltage protection circuitry. Thus, what is needed are electric circuit and transducer topologies such that low noise, low voltage integrated receive circuits can withstand transmit voltages.
Ultimately what is needed is a solution to the problem for operating a capacitive microfabricated transducer efficiently and with reduced acoustic reflectivity.